Throughout their lives, plants are routinely subjected to a variety of stresses which act to impede or alter growth and development processes. Stress to the growth and development of agricultural plants has a negative economic impact in the form of reduced yields, increased expenditures to ameliorate the effects of stress, or both. Given the world's increasing human population and the diminishing land area available for agriculture, improving agricultural productivity is of paramount importance. Thus, there is a need for crop plants that are better able to tolerate stresses and maintain productivity under unfavorable conditions.
While traditional plant breeding approaches will continue to be important for improving agricultural plants, the new strategies that are likely to have the most significant impact on crop improvement will involve genetic engineering. A thorough understanding of the molecular and cellular mechanisms used by plants to avoid or tolerate stresses will aid in the development of new strategies to improve the stress tolerance of agricultural plants.
Stresses to plants may be caused by both biotic and abiotic agents. For example, biotic causes of stress include infection with a pathogen, insect feeding, parasitism by another plant such as mistletoe, and grazing by animals. Abiotic stresses include, for example, excessive or insufficient available water, insufficient light intensity, temperature extremes, synthetic chemicals such as herbicides, and excessive wind. Yet plants survive and often flourish, even under unfavorable conditions, using a variety of internal and external mechanisms for avoiding or tolerating stress. Plants' physiological responses to stress reflect changes in gene expression.
Grain yield in Zea mays is dependent upon the number of ovaries which are initiated, are fertilized, and develop to maturity. Reduced grain production may result from, inter alia, a decrease in the number of kernel initials, restricted or untimely silk exsertion, and/or kernel abortion during grain development.
Maize silks comprise the stigmatic tissues of the flower, intercepting air-borne pollen and supporting pollen tube growth to result in fertilization. Silk receptivity to pollen is limited in duration and is affected by environmental factors. For example, under drought conditions, silk exsertion is delayed or restricted and thus may not occur at the proper time relative to pollen shed. (See, for example, Herrero, M. P., and R. R. Johnson, Crop Science 21:105-110, 1981.) Importantly, the process of fertilization determines kernel number and thus sets an irreversible upper limit on grain yield.
What is needed in the art is a means to stabilize yield of maize across environments by ensuring ample and timely silk exsertion. This can be accomplished through transgenic modifications to create a plant with constant or increased rates of silk exsertion, even under stress, relative to an unmodified plant.
Modification of gene expression affecting silk growth and development requires use of promoters expressed exclusively or preferentially in silk tissues; for example, see U.S. Pat. No. 6,515,204. Also needed are coding regions capable of enhancing silk growth and development. The present invention meets these and other objectives.